# How to read an ACDelco spark plug reference

> ACDelco identifies plugs by a part number plus a specification sheet (dimensions). Here is how to read the code and the ranges.

An ACDelco code such as **R44XLS** or **41-101** combines a resistor mark, a heat/reach class and design-feature letters. ACDelco also publishes the exact dimensions (reach, gap, insulator diameter) per part number.

## 1. R — resistor

An **R** at the front marks a **resistor-style** plug (interference suppression). Most modern ACDelco plugs are resistor type.

## 2. The number — heat / reach class

The digits identify the plug type (heat range and reach class) within the range. Compare with the equivalents on the page to confirm the exact fitment.

## 3. Suffix letters — design features

Trailing letters (e.g. **X**, **L**, **S**, **C**) mark design features such as projected tip, extended/long reach, special gap or copper core.

## 4. Ranges

- **Professional** — conventional nickel / copper core
- **Platinum** / **Double Platinum** — longer life
- **Iridium** / **Finewire** — finest tip, longest life

## 5. Dimensions

ACDelco lists, per part number: insulator barrel diameter, installed height, **reach** (mm and inch), spark location, **gap** and terminal type. Unlike NGK or Champion, ACDelco does not use a strict letter-by-letter key — always confirm the fitment with the listed equivalents.

## 6. Tightening torque

Torque depends on the seat, thread diameter and cylinder-head material. Flat seat (with gasket): 12 mm 15–25 N·m (iron) / 15–20 (alu), 14 mm 25–35 / 25–30, 18 mm 35–45 / 35–40. Taper seat: 12–14 mm 10–20 N·m. Always follow the value printed on the box.

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*Source : [How to read an ACDelco spark plug reference](https://www.thesparktwin.com/en/guide/nomenclature-acdelco/) — TheSparkTwin*